class_docstr: Samples elements from a Dataset for which `fn` returns True.
  
  Parameters
  ----------
  fn : callable
      A callable function that takes a sample and returns a boolean
  dataset : Dataset
      The dataset to filter.
code: def __len__(self):
      return len(self._indices)
  
imports: import numpy
label_desc: Use image node_id 3 for calling the FilterSampler obj's underlying member method code with example usage: obj.__len__() and returns: len